In order to increase the life of spark plugs the center electrodes thereof have been provided with firing tips of corrosion and oxidation resistant metals, such as those of the platinum metal family, i.e., gold, osmium, iridium, ruthenium, palladium, rhodium and platinum, platinum and gold being preferred.
Since these metals are expensive, only the firing tip is composed of them, the tip being attached to the lower end of the center electrode, the balance of which consists of some less expensive metal such as nickel or the like.
The joining of the firing tip to the center electrode is typically accomplished by welding, such as resistance welding, brazing and the like. Other joining techniques, such as swaging or other mechanical locking, may also be used. Any mode which provides a sound bond and which is electrically conductive is satisfactory for forming the joint.
A problem which has arisen at these joints or interfaces is the oxidation and corrosion attack thereat which causes partial and sometimes complete separation of the tip from the rest of the electrode body. This in part is due to the corrosive action resulting from the combustion process. The result is a decrease in or loss of the conduction of electricity to the tip and a voltage drop which affects the function of the plug.